This invention relates to a method for preparing catalysts useful in the production of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly it is directed to the preparation of catalysts suitable for producing maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or a mixture thereof.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve combining a vanadium compound, a phosphorus compound, and if desired, promoter element compounds in a reducing medium under conditions which will provide vanadium in a valence state below +5 to form catalyst precursors capable of being converted to an oxide. The catalyst oxide precursor is then recovered and calcined to provide active catalytic material.
The use of gaseous HCl as a reducing agent for vanadium is disclosed in U.S. Pat. No. 4,002,650 where the vanadium and phosphorus components are present in an aqueous solution. The use of gaseous HCl as a reducing agent for vanadium is also described in U.S. Pat. No. 4,043,943 where the vanadium and phosphorus components are present in a liquid organic medium.
U.S. Pat. No. 4,016,105 describes the preparation vanadium and phosphorus oxide-containing catalysts, utilizing as reducing agents, organic acids or aldehydes, together with a co-reducing secondary alcohol. These reducing agents are added to an aqueous solution containing the vanadium and phosphorus components.
Similar preparational techniques are described in European Patent Appln. No. 3,431 in which the additional step of comminuting the vanadium-phosphorus precursor to a particle size of 500 to 700 microns (0.5 to 0.7 mm) is disclosed.
The use of such reducing agents as disclosed in the art, requires special precautions in the preparation of these catalysts because of the corrosive nature of the materials utilized.
A method for preparing catalysts containing vanadium and phosphorus oxides was described in U.S. Pat. No. 4,132,670 which required the maintenance of a solid phase and dispersion of the vanadium-containing feed compound. The method includes forming a vanadium-containing compound dispersion in an organic liquid medium such as alcohols, aldehydes, ketones, ethers or mixtures thereof, heating the dispersion to reduce the vanadium, and adding phosphoric acid in an organic solvent.
In the methods described above, separation of the catalyst precursor from the reaction solution has provided difficulties. Conventionally, the solution containing the precursor must be evaporated down, usually to a catalyst precursor-containing paste which must then be dried, broken up and ground. This provides difficulties for the commercial scale- up of the process, particularly where the catalyst precursor-containing solution includes flammable organic liquids. Where the solid phase dispersion has been maintained throughout the reduction of the vanadium and reaction with phosphoric acid, separation is more easily effected. In this instance, however, the catalyst obtained requires high operating temperatures when used to produce maleic anhydride, and produces modest yields of product.